The present invention relates to an automatic speed gear for bicycles and the like.
As is known, travel and sports bicycles are provided with a pinion assembly, including a plurality of pinions having a different number of teeth, which are applied to the hub of the bicycle rear wheel.
These pinions can be selectively coupled to a driving pinion, which is driven by the bicycle pedals, through the chain, and the switching over of the chain from one pinion of the assembly to another pinion thereof, is obtained by a chain-guide member, which is supported by the pin or axle of the rear wheel, near the pinion assembly.
The chain-guide member, which conventionally also comprises a chain sretcher, can be displaced with a parallel relationship with respect to the axle of the rear wheel, in order to cause the chain to be disengaged from a pinion and to be engaged with another pinion.
The displacement of the chain-guide member is transmitted by a lever, which can be manually operated, arranged on the bicycle handlebar and coupled to the chain guide member by means of a Bowden cable.
The speed gear, by changing the transmission ratio between the driving pinion and the rear wheel, allows a cyclist to reduce his/her effort in order to travel on slopes, or allows the cyclist to achieve quick accelerations and high speed in driving the bicycle.
In the most convenient speed gears for bicycles, the user, in order to operate the speed gear, is compelled to remove one of his/her hands from the handlebar, in order to operate the control lever.
Such an operation can cause dangerous skiddings of the cyclist which can have serious consequences, especially on a high traffic road.
Moreover, as a cyclist must travel on a slope, he must choose a priori the optimum transmission ratio allowing the cyclist to get over the slope without excessively reduce the travel speed.
This operation, which is easy for skilled cyclists, is comparatively difficult for unskilled cyclists who must try several gear ratios before finding the optimal ratio.
In this connection it should be pointed out that the actuation of the speed gear as a cyclist is exerting a comparatively great effort on the pedals may be dangerous, both with respect to the equilibrium of the cyclist and with respect to the preservation of a sound condition of the speed gear.
Thus, the aim of the present invention is to overcome the above mentioned drawbacks, by providing a speed gear for bicycles and the like which can be automatically controlled.
In particular, the speed gear according to the invention, which is adapted to automatically change the transmission or gear ratio, depending on the effort exerted by the cyclist on the bicycle pedal, allows the effort to be specifically fitted to the physical power of the user.
According to one aspect of the present invention, the above mentioned aim, as well as the above mentioned objects and other objects, which will become more apparent hereinafter, are achieved by an automatic speed gear for bicycles, or the like, comprising a driving pinion, rotatably rigid with a main driving shaft, which can be driven with a rotary motion about a rotary axis thereof by a pair of pedals, and an assembly of driven pinions, said driven pinions having different numbers of teeth, and being coupled to the hub of a wheel of the bicycle and being adapted to be selectively connected to the driving pinion by means of a chain, a chain guide member being moreover provided, which operates on the chain near the driven pinions, for causing the chain to be switched over from one of the pinions to another of said pinion, thereby changing the transmission or gear ratio, characterized in that it comprises a device for controlling the chain guide member, mounted on said shaft and including an element adapted to be axially displaced along the main shaft, as counter-biassed by resilient means, driving means being moreover provided affecting said movable element to displace it, against the biassing of said resilient means, as on said main shaft there is exceeded a preset torque effort, the movable element being coupled to the chain-guide member in order to cause the chain to be switched over from a driving pinion to another driven pinion, providing a less transmission ratio, as the preset torque effort is exceeded.